TECHNICAL
Chamber correction EQ
Physical Design
The physical design of a Tetrahedral Test Chamber, is based upon the triangle which is amongst the strongest known structures; also differs from test boxes. By minimising parallel surfaces, these in turn reduce modal effects that seriously interfere with measurement chambers.
The measurement microphone is fitted inside the chamber at a known and easily set distance. The loudspeaker to be tested is placed so that it's sound is directed into the chamber - towards the microphone, not out of it like a normal loudspeaker enclosure.
Individual sub baffles are then used to adapt individual loudspeaker drive units to the test chambers allowing rapid and very accurate physical location which equates to more stable measurements.
We have experience integrating TTC's into production environments.
How Accurate and How Consistent is a TTC?
The top graph shows the measurements of a single SEAS loudspeaker drive unit #7 measured fifty times with the chamber equalisation applied to a TTC900. The one-third octave curve is the error level less than 0.2dB at 95% confidence level, this from 20Hz to 10kHz, even at the loudspeaker drivers resonance! Up to 20kHz where the SPL drops off.
The lower graph shows the results of a seriers of these loudspeaker drive units #2 to #7 but measured sixty eight times. This shows greater variation as would be expected from different loudspeakers. Note: The minimum variance shown in the mid band is 0.25dB, is in agreement with the first graph, so these are genuine loudspeaker response variations.
These graphs demonstrates the control and stability of the measurements that a tetrahedral test chamber can provide, wherever they are used. R&D, QC, production, goods in or repair and refurbishment all can benefit equally.
Consistently Stable Loudspeaker Measurements using a Tetrahedral Enclosure, AES 135th Convention, 2013 October 17-20 New York, USA
Comparative Results between Loudspeaker Measurements Using a Tetrahedral Enclosure and other Methods, AES 136th Convention, 2014 April 26-29 Berlin
TTC350 on the cover and in Voice Coil Magazine, November 2014
What is a Tetrahedral Test Chamber? - 2015
Hill Acoustics Tetrahedral Test Chamber Whitepaper - 2015
Loudspeaker Modelling and Design A Practical Introduction - Geoff Hill, published by Routledge 2019
Publications & Articles
How can you create an accurate chamber correction curve and how can you KNOW it is accurate?
This is probably the number one question for a new user who is used to either anechoic or free field conditions.
Most loudspeakers are described as "Pistonic" which means at low frequencies below the cone break-up region the whole diaphragm moves as a single mass or entity, so the sound pressure created at the front of the diapragm is and must be the same as at the back of the diaphragm.
We can use this to accurately calculate the difference between the internal pressure response and the near field external response and this allows us to create an accurate chamber correction curve.
The next step is to ensure that the chamber interfere's with the measurements as little as possible and this is covered by the physical design of the TTC.